1. Field of the Invention
This invention relates to the field of purification of fluids, and more specifically to the removal of trace contaminants from inert, non-reactive gases and reactive fluids using solid scavenger adsorption materials, without concurrently emitting water vapor or other contaminants into the gas stream. More particularly, this invention provides methods for reducing concentrations of trace contaminants in inert and non-reactive gases to parts-per-billion (ppb) and sub-parts-per-billion (sub-ppb) levels using an ultra-low emission carbon based scavenger, wherein the impurities include carbon monoxide, carbon dioxide, and organic compounds such as hydrocarbons. This invention further provides methods for reducing concentrations of trace impurities in reactive fluids to parts-per-billion and sub-parts-per-billion levels using a preconditioned ultra-low emission carbon based scavenger, wherein the impurities include carbon monoxide, carbon dioxide, and organic compounds such as hydrocarbons.
2. Description of the Prior Art
Inert and non-reactive gases such as nitrogen, helium, and argon are widely used in the semiconductor industry for the manufacture of microcircuitry devices. In such applications, it is critical that the gases be essentially completely free of impurities such as water and oxygen. For example, in semiconductor fabrication processes, gases such as nitrogen, helium and argon are often required to not have more than low ppb or sub-ppb impurity levels to ensure that the impurities do not degrade the quality, and hence the performance of the semiconductor chips. Such impurities, when introduced onto the semiconductor chip during its manufacture, tend to render the chip deficient or even useless for its intended purpose. Thus, a growing number of industries are now requiring gases having impurity concentrations that do not exceed about 10 parts-per-billion (ppb) levels.
In addition, semiconductor fabrication processes use reactive gases, including dry-etch gases such as hydrogen chloride, hydrogen bromide, chlorine and silicon tetrachloride, and production gases such as arsine and phosphine, and ammonia, which is a precursor of nitride semiconductor materials such as gallium nitride, silicon nitride, and indium nitride. These electronic reactive gases are often required to not have more than low ppb or sub-ppb impurity levels to ensure that the impurities do not degrade the quality, and hence the performance of the semiconductors produced or treated by those gases. Specifically, the semiconductor industry requires ammonia gas (NH3) to have the purity level of xe2x80x9csuperammonia,xe2x80x9d a term of art used to describe ammonia gas that does not contain more than about 1 ppb level impurities. While moisture is usually the main contaminant in high-purity ammonia, other impurities may also exist in ammonia gas such as oxygen, carbon oxides, and volatile organicsxe2x80x94especially lower hydrocarbons such as volatile alkanes. In some cases, ammonia gas may accommodate amines and sulfur-containing molecular impurities. Thus, gas purification systems are widely used in the manufacture of semiconductors to remove process gas impurities to very low, trace concentrations.
The desire to develop methods to reduce impurities in process gases down to sub-part-per-million (sub-ppm) or sub-ppb concentrations is further driven by the present ability to measure impurities at extremely low levels. Modem analytical instrumentation such as Fourier Transform Infra Red Spectrometry (FTIR) and Gas Chromatography-Pulsed Discharge Helium Ionization Detector (GC-PDHID) permits the detection of process gas impurities such as carbon monoxide, carbon dioxide, oxygen, and moisture (H2O) at sub-ppm concentrations, down to about 10 ppb. Atmospheric Pressure Ion Mass Spectrometry (APIMS) permits detection of contaminants in inert and non-reactive gases, such as nitrogen and argon, in the 10-100 parts per trillion (ppt) range.
The advances in the detection of trace levels of hydrocarbons using the above-described analytical instrumentation has motivated researchers to further reduce the levels of these impurities in ultra-pure process gases to below the limits of detection of these ultrasensitive instrumentations. One challenge has been to develop gas purification materials and techniques that remove hydrocarbon impurities from an ultra-pure gas without adding trace amounts of other impurities.
One known method of gas purification involves the adsorption of process gas impurities on a bed or column of solid scavenger material. In these solid adsorption methods, impurities are caught by the surface of the scavenger material while the process gas preferably passes unaltered through the bed or column. Commonly used solid scavenger adsorption materials include alumina, silica, silica-alumina, other metal oxides such as titania and zirconia, mixed oxides, clays, molecular sieves (e.g., zeolites), and activated carbon. Activated carbon, for example, is used in PSA (Pressure Swing Adsorption) plants and for solvent recovery from air in painting facilities (See, for example, Wood and Stampfer, Carbon, 30:593 (1992); Wood and Stampfer, Carbon, 31:195 (1993); Nelson et al., Am. Ind. Hyg. Assoc. J., 33:797 (1972); and Nelson et al., Am. Ind. Hyg. Assoc. J., 52:235 (1991)). However, the use of solid scavenger adsorption materials operating at ambient conditions to reduce low parts-per-million (ppm) or high parts-per-billion (ppb) levels of impurities, particularly hydrocarbons, to low ppb or sub-ppb levels without contaminating the gas stream with other impurities, such as moisture, is not known.
Conventionally activated carbon, for example, is known as a very effective adsorbent for removing hydrocarbon impurities from gases. However, conventionally activated carbon is typically activated at 200xc2x0 C. to 400xc2x0 C. in gas streams contaminated with ppm levels of impurities such as moisture and CO2. After conventional activation, the carbon material contains trace amounts of water and CO2 that are either not completely removed during activation or re-adsorbed in the contaminated environment of the treatment process. The carbon material may also produce trace amounts of moisture and CO2 during thermal activation due to chemical reaction of residual functional groups or adsorbed species, such as by dehydroxylation or decarboxylation reactions. Furthermore, gas impurities such as moisture may be generated upon contacting conventionally activated carbon material with reactive gases, through reactions of the reactive gas with surface impurities in the carbon. The residual water and CO2 in the conventionally activated carbon material are then released in small quantities into a gas stream during a gas purification process, thereby causing significant contamination of the gas and rendering the effluent gas useless for high purity applications. In some cases, conventionally activated carbon is characterized as xe2x80x9chydrophobicxe2x80x9d (repels or fails to adsorb water), even though in some cases activated carbon has been shown to weakly adsorb moisture upon exposure of a gas containing several hundreds to several thousands of ppm of moisture (see, for example, Barton et al., Carbon, 22:22 (1984). However, this adsorbed moisture is also easily released into a process gas stream during purification of the gas. Thus, reducing hydrocarbon impurities in a process gas to sub-ppb levels while maintaining very low levels of water vapor and CO2 has proven extremely difficult.
Among the methods utilized in the prior art for removing water from ammonia is the use of moisture-sorptive molecular sieves. The difficulty of employing such method for the production of high-purity ammonia for semiconductor applications is that ammonia is competitive with water for the adsorption sites on the molecular sieves. As a result, it is not possible to obtain the necessary low residual water values, on the order of part- per-billion concentrations of water in the effluent, using conventionally activated molecular sieves.
JP10297919A2 to Nissan Chemical Industry Ltd., discloses a process for purifying ammonia water by evaporation to liquid ammonia, subjecting the liquid ammonia to adsorption treatment over activated carbon, distilling the ammonia under pressure, and finally introducing ultra-pure water into the purified liquid ammonia at a desired ratio.
JP6024737A2 to Iwatani International Corporation describes the elimination of carbon dioxide impurity from ammonia gas by passing the ammonia gas through a solid alkali layer and removing the impurity by adsorption.
JP55090419A2 to Daikin Ind. Ltd., discloses selective removal of sulfur compounds such as mercaptans from ammonia by adsorption over an activated carbon.
None of the above-described patents discloses an effective purification method for ammonia gas based on adsorption on activated carbon to remove impurities in the ammonia gas to a level of below about 100 ppb, while not concurrently adding impurities such as moisture at ppm levels or higher into the purified ammonia gas stream.
U.S. Pat. No. 5,704,965 to Tom et al. teaches a method for storing and dispensing sorbable gases such as ammonia, silane, germane, arsine, and phosphine, comprising physically sorptively loading the gas on a carbon sorbent material, wherein the gas is physically adsorbed by the pores, surfaces, and microcavities of the carbon sorbent material. U.S. Pat. No. 5,704,965 does not teach using the carbon material to reduce concentrations of trace impurities from the gases.
Accordingly, one aspect of this invention is to provide a method for reducing the concentration of hydrocarbon impurities as well as other contaminants in an inert or non-reactive process gas to levels on the order of sub-parts-per-billion (sub-ppb), without concurrently emitting higher levels of other contaminants, such as water vapor and CO2, into the inert and non-reactive process gas being purified.
More specifically, this invention provides a method for producing an ultra low emission carbon material, referred to herein as a xe2x80x9cULExe2x80x9d carbon material, for purifying inert and non-reactive gases, comprising:
a) heating a carbon material under inert conditions at a temperature and for a time sufficient to remove substantially all of the water and carbon dioxide (CO2) contained in the carbon material to produce a ULE carbon material, and
b) transferring the ULE carbon material to a container under conditions that do not allow moisture, carbon dioxide, or other atmospheric contaminants to be reintroduced into the ULE carbon material.
This invention further provides ULE carbon materials, referred to herein as xe2x80x9cULExe2x80x9d carbon materials, for reducing concentrations of trace impurities (contaminants) in inert and non-reactive process gas streams such as helium (He), nitrogen (N2) and argon (Ar) to levels on the order of parts-per-billion (ppb) and sub-parts-per-billion (sub-ppb), wherein the impurities include, but are not limited to, carbon monoxide (CO), carbon dioxide (CO2), small amounts of water vapor, and organic compounds including, but not limited to, hydrocarbons.
This invention further provides a one-component gas purifier system comprising a bed of a ULE carbon material of this invention, wherein the one-component gas purifier system is capable of reducing concentrations of trace impurities in inert or nonreactive process gases to levels on the order of ppb and sub-ppb levels, wherein the impurities include, but are not limited to, carbon monoxide, carbon dioxide, water vapor, and organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons.
This invention further provides a two-component gas purifier system for purifying inert and non-reactive gases, wherein the purifier comprises a ULE carbon material of this invention and a secondary scavenger material capable of removing impurities such as oxygen and larger quantities of moisture that are not scavenged by the ULE carbon material. The secondary purifier material is referred to herein as a xe2x80x9csecondary scavenger.xe2x80x9d The two-component purifier system of this invention acts as a combination gas purifier capable of producing a purified inert or non-reactive gas with only sub-ppb levels of impurities, such as carbon monoxide, carbon dioxide, oxygen, water vapor, and organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons.
This invention further provides a method for reducing the concentration of hydrocarbon impurities as well as other impurities in reactive fluids, e.g., a gas, vapor, liquid, multiphase fluid, etc., to levels on the order of sub-parts-per-billion (sub-ppb), without concurrently emitting very low levels of other contaminants such as water vapor and CO2 into the reactive fluid being purified.
More specifically, this invention provides a method for producing a preconditioned ultra-low emission carbon material, referred to herein as P-ULE carbon materials, for purifying reactive fluids, comprising:
a) heating a carbon material under inert conditions at a temperature and for a time sufficient to remove substantially all of the water and carbon dioxide (CO2) contained in the carbon material to produce a ULE carbon material,
b) preconditioning the ULE carbon material by the method comprising:
i) purging the ULE carbon material with an ultra-purified reactive fluid at room temperature for a specific period of time,
ii) heating the ULE carbon material under the ultra-purified reactive fluid purge at a temperature range of about 50xc2x0 to 400xc2x0 C. for between a few hours and a few days, thereby producing a P-ULE carbon material,
iii) cooling to ambient temperature, and
c) transferring the P-ULE carbon material to a container under conditions that do not allow moisture, carbon dioxide, or other atmospheric contaminants to be reintroduced into the P-ULE carbon material.
This invention further provides xe2x80x9cpreconditioned ultra-low emissionxe2x80x9d (P-ULE) carbon materials for reducing concentrations of trace impurities in reactive fluids such as ammonia (NH3), hydrogen chloride (HCl), hydrogen bromide (HBr) and chlorine (Cl2) to levels on the order of parts-per-billion (ppb) and sub-parts-per-billion (sub-ppb), wherein the impurities may include (for example, CO2 impurity is not removed from HBr), but are not limited to, carbon monoxide (CO), carbon dioxide (CO2), small amounts of water vapor (H2O) and organic compounds including, but not limited to, hydrocarbons.
This invention further provides a one-component reactive gas purifier system comprising a bed of a P-ULE carbon material of this invention, wherein the one-component gas purifier system is capable of reducing trace amounts of impurities in reactive fluids to levels on the order of parts-per billion (ppb) and sub-parts-per-billion (sub-ppb), wherein the impurities may include carbon monoxide (CO), carbon dioxide (CO2), small amounts of water vapor (H2O) and organic compounds including, but not limited to, hydrocarbons.
This invention further provides a two-component reactive gas purifier system for purifying reactive fluids, wherein the purifier comprises a P-ULE carbon material of this invention and a secondary scavenger material capable of removing impurities such as oxygen and larger quantities of moisture that are not scavenged by the P-ULE carbon material. The two-component reactive fluid purifier system of this invention therefore acts as a combination reactive gas purifier where the P-ULE carbon removes CO, CO2, and volatile organic compounds such as hydrocarbons, and the secondary scavenger removes moisture and/or O2, to provide a reactive gas stream wherein all the above impurities are reduced.
Additional novel features of this invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following specification or may be learned by the practice of the invention. The novel features of the invention may be realized and attained by means of the instrumentalities, combinations, and methods particularly pointed out in the appended claims.